KEYWORDS: Space operations, Telescopes, Signal processing, Receivers, Transmitters, Laser safety, Interfaces, Sensors, Deep space optical communications, Actuators
The National Aeronautics and Space Administration’s (NASA) Deep Space Optical Communications (DSOC) payload, launched with the Psyche spacecraft on October 13, 2023, is facilitating an ongoing Technology Demonstration (TD) of Free-Space Optical Communications (FSOC), from beyond the earth-moon system. The DSOC Flight Laser Transceiver (FLT), can acquire a 1064 nm uplink laser from earth, and return a 1550 nm, Serially Concatenated Pulse Position Modulated (SCPPM) signal, to earth. The FLT uses a 22 cm diameter unobscured optical transceiver assembly, coupled to a 4 W average power laser transmitter, supplemented with actuators, sensors, electronics and software. A 5-7 kW average power, multi-beam 1064 nm uplink laser assembly integrated to the Optical Communications Telescope Laboratory (OCTL) near Wrightwood, CA serves as the Ground Laser Transmitter (GLT). The DSOC Ground Laser Receiver (GLR) at the Palomar Observatory, Hale telescope (operated by Caltech Optical Observatories), consists of a Superconducting Nanowire Single Photon Detector (SNSPD) array, connected to a ground signal processing assembly. Signal photon arrivals are detected and processed to extract information codewords at the GLR. A Mission Operations System (MOS) co-located with the Psyche Project Mission Operations Center, at the Jet Propulsion Laboratory (JPL), coordinates DSOC technology demonstration activities. This paper presents a system overview, mission description and operations architecture for the TD. Early results that include downlink at maximum downlink data-rate of 267 Mb/s from 0.37 Astronomical Units (AU) or 55 million kilometers are presented.
The MSTAR sensor (Modulation Sideband Technology for Absolute Ranging) is a new system for measuring absolute distance, capable of resolving the integer cycle ambiguity of standard interferometers, and making it possible to measure distance with subnanometer accuracy. The sensor uses a single laser in conjunction with fast phase modulators and low frequency detectors. We describe the design of the system - the principle of operation, the metrology source, beam-launching optics, and signal processing - and show results for target distances up to 1 meter. We then demonstrate how the system can be scaled to kilometer-scale distances and used for space-based applications.
This paper provides an overview of technology development for the Terrestrial Planet Finder Interferometer (TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like planets in the habitable zones around nearby stars.
Large aperture telescope commonly features segment mirrors and a coarse phasing step is needed
to bring these individual segments into the fine phasing capture range. Dispersed Fringe Sensing
(DFS) is a powerful coarse phasing technique and its alteration is currently being used for JWST.
An Advanced Dispersed Fringe Sensing (ADFS) algorithm is recently developed to improve the
performance and robustness of previous DFS algorithms with better accuracy and unique
solution. The first part of the paper introduces the basic ideas and the essential features of the
ADFS algorithm and presents the some algorithm sensitivity study results. The second part of the
paper describes the full details of algorithm validation process through the advanced wavefront
sensing and correction testbed (AWCT): first, the optimization of the DFS hardware of AWCT
to ensure the data accuracy and reliability is illustrated. Then, a few carefully designed algorithm
validation experiments are implemented, and the corresponding data analysis results are shown.
Finally the fiducial calibration using Range-Gate-Metrology technique is carried out and a
<10nm or <1% algorithm accuracy is demonstrated.
KEYWORDS: Nulling interferometry, Planets, Interferometers, Space operations, Mirrors, Robots, Control systems, Stars, Space telescopes, Interferometry
The last decade has seen great advances in interferometric nulling technology, propelled at first by the SIM and KECK
nulling programs and then by the Terrestrial Planet Finder Interferometer (TPF-I). In the infrared at N-band (using a CO2
laser at 10.6 micron wavelength) the first million to one nulls were reported on a KECK testbed in 2003. For TPF-I,
nulls needed to be both deep and broadband, and a suite of testbeds was designed and built to study all aspects of
achromatic nulling and system implementation, including formation flying technology. Also, observatory designs were
drawn up and studied against performance models. Modeling revealed that natural variations in the alignment and
control of the optical system produced an "instability noise" signal and this realization eventually led to a redesign of the
layout to a rectangular formation. The complexity of the early TPF-I spacecraft design was mitigated by the infusion of
ideas from Europe and produced the current X-Array design which utilizes simple reflectors to form the apertures
together with a stretched three dimensional formation geometry. This paper summarizes the main achievements of the
infrared nulling technology program including the development of adaptive nulling for broadband performance and the
demonstration of starlight suppression by 100 million to one.
Modal filters are necessary to the proposed high-performance mid-infrared nulling interferometers, because they can
help achieve deeper interferometric nulls. Silver halide fibers of composition AgClxBr1-x(0
This paper provides an overview of technology development for the Terrestrial Planet Finder Interferometer
(TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like
planets in the habitable zones around nearby stars. The overall technology roadmap is presented and progress
with each of the testbeds is summarized.
SCDU (Spectral Calibration Development Unit) is a vacuum test bed that was built and operated for the SIM-Planetquest
Mission and has successfully demonstrated the calibration of spectral instrument error to an accuracy of
better than 20 picometers. This performance is consistent with the 1 micro-arc second goal of SIM. The calibration
procedure demonstrated in the test bed is traceable to the SIM flight instrument. This article is a review of all aspects of
the design and operation of the hardware as well as the methodology for spectral calibration. Spectral calibration to
better than 20 picometers and implications for flight are discussed.
With the launch of planet-transit missions such as CoRoT and Kepler, it is expected that Earth-sized planets
orbiting distant stars will be detected soon. This milestone will open the path towards the definition of missions
able to study the atmosphere of Earth-sized extrasolar planets, with the identification of bio-signatures as one of
the main objectives. In that respect, both the European Space Agency (ESA) and the National Aeronautics and
Space Administration (NASA) have identified nulling interferometry as one of the most promising techniques.
Trying to minimize the cost and the technological risks while maximizing the scientific return, ESA and NASA
recently converged towards a single mission architecture, the Emma X-array. In this paper, we present the
expected science performance of this concept computed with two independent mission simulators. The impact of
different observational parameters such as planet radius and exozodiacal cloud density is specifically addressed.
Deep, stable starlight suppression is needed for the direct interferometric detection of Earth-like planets and
requires careful control of the intensity and phase of the beams that are being combined. We have
developed a novel compensator for the Terrestrial Planet Finder Interferometer based on a deformable
mirror to correct the intensity and phase at each wavelength across the bandwidth of 8 to 12 microns
wavelength. This paper will discuss the results of using the adaptive nuller to achieve deep broadband
nulling in the mid-IR.
Point-to-point laser metrology systems can be used to stabilize large structures at the nanometer levels required for
precision optical systems. Existing sensors are large and intrusive, however, with optical heads that consist of several
optical elements and require multiple optical fiber connections. The use of point-to-point laser metrology has therefore
been limited to applications where only a few gauges are needed and there is sufficient space to accommodate them.
Range-Gated Metrology is a signal processing technique that preserves nanometer-level or better performance while
enabling: (1) a greatly simplified optical head - a single fiber optic collimator - that can be made very compact, and (2) a
single optical fiber connection that is readily multiplexed. This combination of features means that it will be
straightforward and cost-effective to embed tens or hundreds of compact metrology gauges to stabilize a large structure.
In this paper we describe the concept behind Range-Gated Metrology, demonstrate the performance in a laboratory
environment, and give examples of how such a sensor system might be deployed.
A novel space interferometer design originating in Europe has been studied. The interferometer uses the technique of
starlight nulling to enable detection of earth-like planets orbiting nearby stars. A set of four telescope spacecraft flying in
formation with a fifth, beam-combiner spacecraft forms the interferometer. This particular concept shows potential for
reducing the mission cost when compared with previous concepts by greatly reducing the complexity of the telescope
spacecraft. These spacecraft have no major deployable systems, have simplified propulsion and a more rugged
construction. The formation flying geometry provides for greater average separation between the spacecraft with
commensurate risk reduction. Key aspects of the design have been studied at the Jet Propulsion Laboratory with a view
to collaborations between NASA and the European Space Agency. An overview of the design study is presented with
some comparisons with the TPF-FFI concept.
This paper provides an overview of technology development for the Terrestrial Planet Finder Interferometer
(TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like
planets in the habitable zones around nearby stars. The overall technology roadmap is presented and progress
with each of the testbeds is summarized. The current interferometer architecture, design trades, and the viability
of possible reduced-scope mission concepts are also presented.
Deep, stable starlight nulls are needed for the direct detection of Earth-like planets and require careful
control of the intensity and phase of the beams that are being combined. We have tested a novel
compensator based on a deformable mirror to correct the intensity and phase at each wavelength across the
bandwidth of 8 to 12 microns wavelength. This paper will cover the results of the adaptive nuller tests
performed in the mid-IR.
The NASA Terrestrial Planet Finder Interferometer (TPF-I) and ESA Darwin missions are designed to directly detect
mid-infrared photons from earth-like planets around nearby stars. Until recently, the baseline TPF-I design was the
planar stretched X-Array, in which the four collectors spacecraft lie on the corners of a rectangle with the combiner
spacecraft at the center, all in the plane normal to the direction to the target star. The stretched X-Array has two major
advantages over other configurations: the angular resolution is very high, and the ability to eliminate instability noise. A
direct consequence of the latter is that the null depth requirement is relaxed from 10-6 to 10-5. Implementation of the
planar configuration requires a significant number of deployments, however, including large sunshades and secondary
mirror supports. ESA had been pursuing a non-planar configuration with 3 collector telescopes. Dubbed the 'Emma'
architecture (after the wife of Charles Darwin), this approach brings the combiner spacecraft up out of the plane of the
collectors, and offers significant simplifications in the collector design with minimal deployments. The Emma X-Array
combines the best aspects of each design, bringing together the 4-collector stretched X-Array collector configuration
with the out-of-plane combiner of the Emma geometry. Both the TPF-I and Darwin missions have now adopted the
Emma X-Array as the baseline design, moving a step closer to a single, joint TPF/Darwin mission.
In this paper we assess the planet-finding performance of the Emma X-Array. An optimized completeness algorithm is
used to estimate the number of Earths that can be found as a function of collector diameter. Other key parameters − the
inner and outer working angles and the angular resolution − are also addressed.
Deep, stable starlight nulls are needed for the direct detection of Earth-like planets and require careful control of the intensity and phases of the beams that are being combined. We are testing a novel compensator based on a deformable mirror to correct the intensity and phase at each wavelength and polarization across the nulling bandwidth. We have successfully demonstrated intensity and phase control using a deformable mirror across a 100nm wide band in the near-IR, and are in the process of conducting experiments in the mid-IR wavelengths. This paper covers the current results and in the mid-IR.
This paper reviews recent progress with technology being developed for the Terrestrial Planet Finder Interferometer (TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like planets in the habitable zones around nearby stars. TPF-I is in the early phase of its development. The science requirements of the mission are described along with the current design of the interferometer. The goals of the nulling and formation-flying testbeds are reviewed. Progress with TPF-I technology milestones are highlighted.
KEYWORDS: Planets, Stars, Space operations, Interferometers, Nulling interferometry, Computer simulations, Interferometry, Robots, Telescopes, Control systems
The interferometric version of the Terrestrial Planet Finder (TPF-I) has the potential to find and characterize earth-sized
planets in the habitable zones of over 250 nearby stars and to search for life using biomarkers in the atmospheres of any
planets found. The scientific case for such a mission continues to be strengthened by on-going progress in the detection
of planets via indirect means. This paper summarizes the status of TPF-I, illustrative scientific requirements for the
mission, and its enabling technologies.
The NASA Terrestrial Planet Finder Interferometer (TPF-I) and ESA Darwin missions are designed to directly detect mid-infrared photons from earth-like planets around nearby stars. The technique of nulling interferometry is used to suppress the light from the parent star, typically 107 times brighter than the planet, with an angular offset of 10-100 mas. There are two classes of noise: photon shot noise from the stellar leakage, local- and exo-zodiacal dust, and instability noise from variations in the instrument response. Shot noise requires that the instrument null depth is at least 10-5. The instability noise requires a null depth of ~10-6, corresponding to control of the optical paths at ~1 nm rms, and control of relative intensities at ~0.2%; it is these requirements that currently drive the design of the instrument. This paper describes a new technique that effectively removes the effect of instability noise. This relaxes the nulling requirement by a factor ~10 and makes planet detection robust to instrument variations. At the same time, the integration period needed to detect and characterize planets is reduced and the angular resolution of the array is significantly improved. Analysis and simulations are presented, and implications for the array architecture are discussed.
Deep, stable starlight nulls are needed for the direct detection of Earth-like planets and require careful control of the intensity and phases of the beams that are being combined. We are testing a novel compensator based on a deformable mirror to correct the intensity and phase at each wavelength and polarization across the nulling bandwidth. We have successfully demonstrated intensity and phase control using a deformable mirror across a 100nm wide band in the near-IR, and are in the process of building the phase 2 experiment operating at mid-IR wavelengths. This paper covers the results of our demonstration in the near-IR, as well as our current progress in the mid-IR.
The Terrestrial Planet Finder Coronagraph (TPF-C) demands extreme wave front control and stability to achieve its goal of detecting earth-like planets around nearby stars. We describe the performance models and error budget used to evaluate image plane contrast and derive engineering requirements for this challenging optical system. We show that when the coronagraph is coupled to an 8th-order band-limited mask, the performance is limited by shearing of the starlight beam across imperfect optics (a.k.a. beam walk), and that this in turn demands tight rigid body pointing, sub-milliarcsecond fine guiding, high-quality optics, and sub-micron positional stability of the optics including the secondary mirror. Additionally we show that the stability of low-order aberrations (focus, astigmatism, coma, and trefoil) is required to be ~ 2-4 Angstroms, while higher-order modes must remain stable to a few picometers.
The direct detection at visible wavelengths of Earth-like planets around nearby stars requires starlight suppression by a factor of 1010 - 1011 at offsets of order 100 mas. It has been shown that perfect suppression is possible in principle, using a combination of a band-limited focal plane coronagraphic mask and a pupil plane Lyot stop. Errors in the transmission amplitude and phase of the mask degrade the performance. These errors can be corrected completely at a given wavelength and polarization using deformable mirrors (DMs) operating in the pupil plane of the system. Both the errors and correction have different chromatic dependences, however, and the DM correction becomes ineffective as the optical bandwidth is increased. The mask errors can be divided into 2 classes: (1) errors that are uncorrelated with the mask pattern, arising, for example, from the surface roughness of the mask substrate, and (2) errors that are correlated with the mask pattern. We present the results of analysis of random errors and simulate the effects of systematic errors using a specific example mask design. In both cases we find that the contrast required by TPF-C imposes very challenging demands on the design and fabrication of the masks. Several potential mitigation approaches are discussed.
The Terrestrial Planet Finder Interferometer (TPF-I) is a space-based NASA mission for the direct detection of Earth-like planets orbiting nearby stars. At the mid-infrared wavelength range of interest, a sun-like star is ~107 times brighter than an earth-like planet, with an angular offset of ~50 mas. A set of formation-flying collector telescopes direct the incoming light to a common location where the beams are combined and detected. The relative locations of the collecting apertures, the way that the beams are routed to the combiner, and the relative amplitudes and phases with which they are combined constitute the architecture of the system. This paper evaluates six of the most promising solutions: the Linear Dual Chopped Bracewell (DCB), X-Array, Diamond DCB, Z-Array, Linear-3 and Triangle architectures.
Each architecture is constrained to fit inside the shroud of a Delta IV Heavy launch vehicle using a parametric model for mass and volume. Both single and dual launch options are considered. The maximum separation between spacecraft is limited by stray light considerations. Given these constraints, the performance of each architecture is assessed by modeling the number of stars that can be surveyed and characterized spectroscopically during the mission lifetime, and by modeling the imaging properties of the configuration and the robustness to failures. The cost and risk for each architecture depends on a number of factors, including the number of launches, and mass margin. Quantitative metrics are used where possible.
A matrix of the architectures and ~30 weighted discriminators was formed. Each architecture was assigned a score for each discriminator. Then the scores were multiplied by the weights and summed to give a total score for each architecture. The X-Array and Linear DCB were judged to be the strongest candidates. The simplicity of the three-collector architectures was not rated to be sufficient to compensate for their reduced performance and increased risk. The decision process is subjective, but transparent and easily adapted to accommodate new architectures and differing priorities.
This overview paper is a progress report about the system design and technology development of two interferometer concepts studied for the Terrestrial Planet Finder (TPF) project. The two concepts are a structurally-connected interferometer (SCI) intended to fulfill minimum TPF science goals and a formation-flying interferometer (FFI) intended to fulfill full science goals. Described are major trades, analyses, and technology experiments completed. Near term plans are also described. This paper covers progress since August 2003 and serves as an update to a paper presented at that month's SPIE conference, "Techniques and Instrumentation for Detection of Exoplanets."
Deep, stable nulling of starlight requires careful control of the amplitudes and phases of the beams that are being combined. The detection of earth-like planets using the interferometer architectures currently being considered require that the electric field amplitudes are balanced at the level of ~ 0.1% and the phases are controlled at the level of 1 mrad (corresponding to ~ 1.5 nm for a wavelength of 10 microns). These conditions must be met simultaneously at all wavelengths across the science band and for both polarization states, imposing unrealistic tolerances on the symmetry between the optical beamtrains. Lay et. al. proposed the concept of a compensator that is inserted into the beamtrain, which can adaptively correct for the mismatches across the spectrum enabling deep nulls with realistic, imperfect optics. This proposed design uses a deformable mirror to adjust the amplitude and phase of the electric field that couples into the single-mode spatial filter. We have demonstrated amplitude and phase control at a single wavelength in the near-IR. We are preparing to demonstrate control with our deformable mirror actuator in the near-IR and in parallel are preparing a demonstration in the mid-IR where the compensator will be required to operate.
Laser metrology systems are a key component of stellar interferometers, used to monitor path lengths and dimensions internal to the instrument. Most interferometers use 'relative' metrology, in which the integer number of wavelengths along the path is unknown, and the measurement of length is ambiguous. Changes in the path length can be measured relative to an initial calibration point, but interruption of the metrology beam at any time requires a re-calibration of the system.
The MSTAR sensor (Modulation Sideband Technology for Absolute Ranging) is a new system for measuring absolute distance, capable of resolving the integer cycle ambiguity of standard interferometers, and making it possible to measure distance with sub-nanometer accuracy. We describe the design of the system, show results for target distances up to 1 meter, and demonstrate how the system can be scaled to kilometer-scale distances. In recent experiments, we have used white light interferometry to augment the 'truth' measurements and validate the zero-point of the system.
MSTAR is a general-purpose tool for conveniently measuring length with much greater accuracy than was previously possible, and has a wide range of possible applications.
A number of stellar systems that can be searched for presence of Earth-like planets in a given mission lifetime is a key figure of merit for planet hunting stellar interferometers. We have developed a method to calculate the number of stellar systems that can be searched and characterized. Using this method we have evaluated the performance of a number of architectures. We conclude that simpler second-order null architectures outperform more complicated fourth-order null architectures. We also quantify the advantages of the variable length formation-flying configurations vs. fixed length structurally connected configurations.
The nulling interferometers proposed for planet detection are arrays of collector telescopes whose amplitudes and phases are carefully controlled to generate a null response at the star. Perturbations in the amplitude and phase response of the instrument lead to time-dependent fluctuations in the stellar leakage that can mimic a planet signal. Understanding these non-linear systematic errors is important, since they drive most of the instrument requirements for missions such as the Terrestrial Planet Finder and Darwin.
We show that 'amplitude-phase' errors are the dominant source of instrument noise. They are unaffected by the technique of phase chopping, increase rapidly at short wavelengths, are largely independent of the size and transmission efficiency of the collector optics, and depend only weakly on the nulling configuration and distance to the target system. Detection of an Earth around a G-type star like the sun requires ~1.5 nm of path control and ~0.1% control of the amplitude, integrated over all frequencies, including DC.
This paper also introduces the X-Array - a new nulling configuration with 4 collectors and a central combiner arranged in an X pattern. This has a number of advantages over the standard dual Bracewell layout, and over other configurations that have been proposed.
Absolute metrology measures the actual distance between two optical fiducials. A number of methods have been employed, including pulsed time-of-flight, intensity-modulated optical beam, and two-color interferometry. The rms accuracy is currently limited to ~5 microns. Resolving the integer number of wavelengths requires a 1-sigma range accuracy of ~0.1 microns. Closing this gap has a large pay-off: the range (length measurement) accuracy can be increased substantially using the unambiguous optical phase.
The MSTAR sensor (Modulation Sideband Technology for Absolute Ranging) is a new system for measuring absolute distance, capable of resolving the integer cycle ambiguity of standard interferometers, and making it possible to measure distance with sub-nanometer accuracy. In this paper, we present recent experiments that use dispersed white light interferometry to independently validate the zero-point of the system. We also describe progress towards reducing the size of optics, and stabilizing the laser wavelength for operation over larger target ranges.
MSTAR is a general-purpose tool for conveniently measuring length with much greater accuracy than was previously possible, and has a wide range of possible applications.
The MSTAR sensor (Modulation Sideband Technology for Absolute Ranging) is a new system for measuring absolute distance, capable of resolving the integer cycle ambiguity of standard intrferometers, and making it possible to measure distance with sub-nanometer accuracy. The sensor uses a single laser in conjugation with fast phase modulators and low-frequency detectors. We describe the design of the system - the principle of operation, the metrology source, beam-launching optics, and signal processing - and show results for target distance up to 1 meter. We then demonstrate how the system can be scaled to kilometer-scale distances.
Deep, stable nulling of starlight requires careful control of the amplitudes and phases of the beams that are being combined. The detection of earth-like planets using the interferometer architectures currently being considered for the Terrestrial Planet Finder mission require that the E-field amplitudes are balanced at the level of ~ 0.1%, and the phases are controlled at the level of 1 mrad (corresponding to ~ 1.5 nm for a wavelength of 10 μm). These conditions must be met simultaneously at all wavelengths across the science band, and for both polarization states, imposing unrealistic
tolerances on the symmetry between the optical beamtrains. We introduce the concept of a compensator that is inserted into the beamtrain, which can adaptively correct for the mismatches across the spectrum, enabling deep nulls with realistic, imperfect optics. The design presented uses a deformable mirror to adjust the amplitude and phase of each beam as an arbitrary function of wavelength and polarization. A proof-of-concept experiment will be conducted at visible / near-IR wavelengths, followed by a system operating in the Mid-IR band.
The TPF interferometer family suppresses the stellar glare using a deep interferometric null, which for the planet can become constructive interference because of its angular offset. The null depth need not be as great as the star-planet contrast, but its systematic fluctuations must be perhaps 5 times better than the variations which constitute the planet's signature. We present an allocation of errors which meet these needs, and identify areas which need better definition.
The StarLight flight project was designed to demonstrate the key technologies of spaceborne long-baseline stellar interferometry and precision formation flying for potential use on the Terrestrial Planet Finder (TPF) and other future astrophysics missions. Interferometer performance validation could be achieved over a 6-12 month period by obtaining several hundred fringe visibility amplitude measurements for stars in the band 600-1000 nm for a variety of stellar visibilities, magnitudes, and baselines. Interferometery could be performed both in a 1 meter fixed-baseline combiner-only mode and in a two-spacecraft formation mode. In formation mode, the combiner spacecraft would remain at the focus of a virtual parabola, while the collector spacecraft assumed various positions along the parabola such that the two arms of the interferometer remained equal over a variety of separations and bearing angles. Challenges to be encountered in flight include high-bandwidth inter-spacecraft stellar and metrology pointing control, alingment and shear correction, delay and delay-rate estimation, visibility calibration, and robust fringe trackign in the presence of local and inter-spacecraft dynamics. This paper is based on the StarLight project design-capture of March 2002 and will describe the StarLight Interferometer System architecture and selected operational concepts.
The StarLight mission is designed to validate the technologies of formation flying and stellar interferometry in space. The mission consists of two spacecraft in an earth-trailing orbit that formation-fly over relative ranges of 40 to 600m to an accuracy of 10 cm. The relative range and bearing of the spacecraft is sensed by a novel RF sensor, the Autonomous Formation Flyer sensor, which provides 2cm and 1mrad range and bearing knowledge between the spacecraft. The spacecraft each host instrument payloads for a Michelson interferometer that exploit the moving spacecraft to generate variable observing baselines between 30 and 125m. The StarLight preliminary design has shown that a formation-flying interferometer involves significant coupling between the major system elements - spacecraft, formation-flying control, formation-flying sensor, and the interferometer instrument. Mission requirements drive innovative approaches for long-range heterodyne metrology, optical design, glint suppression, formation estimation and control, spacecraft design, and mission operation. Experimental results are described for new technology development areas.
There are many advantages to space-based interferometry, but monolithic, single-spacecraft platforms set limits on the collecting area and baseline length. These constraints can be overcome by distributing the optical elements of the interferometer over a system of multiple spacecraft flying in precise formation, opening up new realms of angular resolution and sensitivity. While the principles of interferometry are the same as for structurally-connected systems, formation-flying interferometers must integrate a wide range of technologies to provide an optically stable platform capable of finding, tracking and measuring fringes. This paper discusses some of the key differences between formation-flying and structurally-connected interferometers, including formation configurations, controlling beam shear, station-keeping, and the importance of delay and delay rate estimation in determining the instrument sensitivity.
Proposed future formation-flying interferometer missions include the Terrestrial Planet Finder (TPF), Darwin, the Submillimeter Probe of the Evolution of Cosmic Structure (SPECS), the Stellar Imager, the Micro-Arcsecond Xray Imaging Mission (MAXIM), and its precursor, MAXIM Pathfinder. In addition, Life Finder and Planet Imager have been identified as two formation-flying missions capable of detailed characterization of habitable exo-planets. The parameters for these missions are compared and described briefly.
The Caltech Submillimeter Observatory and the James Clerk Maxwell Telescope have been combined to form the only astronomical interferometer currently operating at submillimeter wavelengths. The telescopes have been operating in this mode for one or two dedicated periods in each of the last 5 years. Results with sub-arcsecond resolution have been obtained at 230, 345 and 460 GHz. The interferometer differs in many ways from the existing millimeter-wave arrays. Connecting two independent telescopes of different design introduces extra problems not encountered with homogeneous arrays of antennas. The CSO-JCMT system is described, with an emphasis on these incompatibility issues and solutions that were adopted. Analysis of data from a single, fixed baseline requires direct modeling of the measured visibilities rather than a synthesized image, an approach that has since proved invaluable for analyzing data from other arrays as well. The sensitivity and angular resolution of the interferometer are limited by fluctuations in the refractive index due to water vapor in the Earth's atmosphere. Two water vapor radiometers have been designed, built and installed to monitor the fluctuations in each beam and generate a correction to the visibility phase measured by the interferometer. These radiometers are described and recent results are presented.
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